Abstract
ABSTRACT:
Passenger car exhaust systems are complex constructions with different sections placing different demands on the materials. One section of the exhaust system requires steels with a high scaling resistance, and the ability to withstand oscillating stresses due to vibration, minimum susceptibility to embrittlement and a low thermal expansion coefficient. In other sections resistance to both high temperature and wet corrosion are needed. In the rear end section the wet corrosion becomes the main factor. Even stainless steels do not possess unlimited corrosion resistance and occasionally these limits are exceeded in the automotive exhaust system. Failure examinations have shown that in the rear, the high-condensation section, the primary cause is the internal attack by chlorideinduced pitting and crevice corrosion. Stainless steels are also subject to pitting and crevice corrosion loads in many other applications. The resistance of different steel grades to this kind of exposure can generally be compared according to their alloy composition via their pitting resistance equivalent number. Of great importance for the special conditions existing in the automotive exhaust system, with their frequent wet/dry alternation and their short operating times, is not only the resistance to the onset of corrosion – as described by the PREN concept – but also the low corrosion rates so as to achieve long lifetime system. To compare the suitability of various stainless steels for use in the wet section of the exhaust system, Konstanz University of Applied Sciences and ThyssenKrupp Nirosta have carried out investigations by simulating these special corrosion conditions. In this paper a corrosion test is proposed which should allow a comparison of materials for applications of this kind. Specimens deep drawn from strip of selected materials are filled with a composition and are then put in an oven at a defined temperature or in a climate chamber at a defined temperature and humidity. A specimen undergoes several cycles of being filled with corrosive medium and drying. The objective is to draw up a material ranking for loads of this kind.